This invention relates to mechanisms for removing integrated circuit packages (IC packages) from a printed circuit board when such devices are soldered to the board.
Typically, between twenty and two hundred integrated circuit packages are soldered to a single board. Thus, when one package fails, it is not economical to simply replace the entire board. Instead, the more common procedure is to remove and replace the failed package on the board.
In the prior art, each integrated circuit package originally had a relatively small number of input/output pins (I/O pins), such as sixteen or twenty-four. Thus, the removal of those chips was a simple task. It was achieved by merely heating the pins one-at-a-time with a soldering iron until the solder melted while simultaneously removing the melted solder with a solder sucker.
To remove the solder from a single pin by the above technique typically takes about twenty seconds. So all of the I/O pins of a sixteen-pin package can be de-soldered in about five minutes.
But with VLSI technology greatly increasing the number of transistors per IC package, there is a need to also greatly increase the number of I/O pins from the package. For example, some recent packages from the assignee of this case have a total of over two hundred pins. Thus the above-described pin-by-pin de-soldering operation is impractical for such packages. At twenty seconds per pin, the de-soldering operation for a two hundred-pin package takes over one hour.
One approach to resolve this problem is to contact the top of a package (rather than the individual pins) with the soldering iron. Then heat is transmitted by conduction from the top of the package through electrical conductors in the package to the I/O pins. However, the conductors in a package are very narrow (e.g., typically less than four mils). Thus, heat conduction from the top of a package to the pins is very poor. Instead, much of the heat from the soldering iron is conducted through the package to its bottom which lies in close proximity to the printed circuit board. From there, the heat is transmitted by convection to the printed circuit board where it can cause damage.
Another approach to resolve the above problem is to provide a solder fountain from which molten solder flows. This flowing solder has a surface area which is slightly larger than the area of the integrated circuit package that is to be removed from the printed circuit board. In operation, the tips of the I/O pins that protrude through the board are placed in the fountain whereupon they heat-up and melt the solder which holds them to the board.
However, all of the I/O pins of an integrated circuit package do not protrude through the printed circuit board by the same distance. This is because the I/O pins are cut at random lengths after they are soldered to the board. Consequently, the solder fountain must be placed very close to the printed circuit board so that the molten solder will contact all of the pins including those of the shortest length. But since the solder fountain and printed circuit board are so close, solder from the fountain can splatter onto the printed circuit board; and this in turn can damage the board.
Further, the cross-sectional area of the tips of the I/O pins is only a small fraction of the area of an integrated circuit package. This is because the I/O pins have spaces between them; and often the pins only border the perimeter of the package. Thus, most of the heat from the solder fountain is not transmitted by conduction to the I/O pins; but instead, most of the heat is transmitted by convection to the printed circuit board beneath the package. This again can cause electrical and mechanical damage to the board.
Accordingly, a primary object of the invention is to provide an improved tool for removing integrated circuit packages, having a large number of I/O pins, that are soldered to a printed circuit board.